A Michelson two-beam interferometer utilized for FTIR (Fourier Transform Infrared Spectroscopy) in a spectrometer is configured to divide, with a beam splitter, measurement light (for example, near-infrared light) in two directions of a fixed mirror and a moving mirror, and to combine, with the beam splitter, light reflected off the fixed mirror and light reflected off the moving mirror into one optical path. When the moving mirror is moved back and forth (in the direction of the optical axis of entrance light), since the optical path difference of the two light beams divided is changed, the combined light becomes measurement interference light (interferogram) whose intensity is changed according to the amount of movement of the moving mirror. This interferogram is sampled and is subjected to AD conversion and Fourier transform, and thus it is possible to determine the spectrum distribution of the entrance light, with the result that it is possible to determine, from the spectrum distribution, the intensity of the measurement interference light per number of waves (1/wavelength).
Since the interferogram described above is indicated by a function of the phase difference of the moving mirror and the fixed mirror, that is, a function of the optical path difference of the light reflected off the moving mirror and the light reflected off the fixed mirror, when the interferogram is sampled and thus its intensity is determined, in general, it is necessary to monitor the position of the moving mirror with a reference light source in addition to a light source that emits the measurement light. Specifically, reference light (for example, laser light) emitted from the reference light source is divided with a beam splitter and guided to the moving mirror and the fixed mirror, the light reflected off the moving mirror and the light reflected off the fixed mirror are combined with the beam splitter and the combined light is guided as reference interference light to a reference light detector for position detection. Since the intensity of the reference interference light is changed according to the position of the moving mirror, variations in the intensity of the reference interference light are detected with the reference light detector, and thus it is possible to determine the position (the optical path difference of the two light beams divided) of the moving mirror.
In this respect, for example, in patent document 1, the optical path of laser light serving as reference light is provided side by side with the optical path of measurement light, and, based on the result of detection of the laser light by a reference light detector, a signal indicating timing at which interferogram is sampled is produced.
In recent years, much attention has been focused on safety and security, and trace detection and high-precision detection have been required in various fields. In particular, in the inspection of medicines in pharmaceutical research laboratories and the like and in hospitals in the medical field, it is important to identify samples with high precision. For example, in order to realize a high-precision measurement, patent document 2 discloses a configuration in which unnecessary light included in measurement light is cut by an optical bandpass filter.